Apparatus for liquid phase epitaxy of mercury cadmium telluride

ABSTRACT

In a covered graphite slider apparatus for the liquid phase epitaxial growth of mercury cadmium telluride this invention shows the addition of a wipe-off well into the moving part of the slider apparatus, which well contains pieces of CdTe to assist wipe-off of HgCdTe following LPE growth.

BACKGROUND AND SUMMARY OF THE INVENTION

The U.S. Government has rights in this invention pursuant to a contractawarded by the Department of Air Force.

The subject matter of this invention generally relates to the field ofapparatus used for the liquid phase epitaxial growth of (Hg_(1-x)Cd_(x))Te films.

The prior art such as U.S. Pat. Nos. 4,317,689 or 4,366,771 (bothassigned to the same assignee as the present application) teaches acovered graphite slider system for LPE growth of HgCdTe. Hg_(1-x) Cd_(x)Te is an important semiconductor for use in photovoltaic andphotoconductive infrared photon detectors. Hg_(1-x) Cd_(x) Te can begrown by liquid phase epitaxy at atmospheric pressure from a Te-richsolution in which case the Hg vapor pressure is below 0.1 atm at 500° C.This low vapor pressure makes possible the use of open-tube, slidergrowth techniques. The teachings of these patents is incorporated byreference.

In FIG. 1, one of the figures is reproduced from the prior art patent. Aquartz tube 10, of which a partial section is shown, provides theconventional furnace enclosure within which the Hg_(1-x) Cd_(x) Te isgrown. A suitable source of heat, such as a heating coil 11 around thetube, provides the desired temperature for the liquid phase epitaxygrowth of Hg_(1-x) Cd_(x) Te. The graphite furnace boat comprises aslider or carrier section 12, a base section 13, and a cover section 14.The slider section 12 has one or more wells 15 extending through thesection, and has a more shallow well or depression 16. The bottomsurface of the slider section 12 has a continuous trough or groove 17therein looping around the bottom near the perimeter. A channelextending in a vertical direction through the slider section connectsthe groove 17 to the top surface. The well 15 is adapted to contain agrowth solution 20 of (Hg_(1-x) Cd_(x))_(1-y) Te_(y). A plug 21 caps thewell. The base section 13 of the boat has a recess 22 in the top surfacethereof. The purpose of the recess is to receive a growth substrate,such as CdTe, upon which the LPE layer of Hg_(1-x) Cd_(x) Te isepitaxially grown. The cover section 14 of the boat has a shoulder 25 onits lower face and extending around its perimeter which is machined tofit closely to the upper surface of the slider section 12.

When growing Hg_(1-x) Cd_(x) Te in the apparatus of FIG. 1, a growthsubstrate is placed in the recess 22. A charge 20 of (Hg_(1-x)Cd_(x))_(1-y) Te_(y) is placed in the well 15 and the well capped withplug 21. It is necessary to prevent loss of Hg from the charge 20 duringthe growth procedure. A Hg source wafer 26 of HgTe plus Te is placed inshallow well 16. The cover 14 is placed over the slider 12 and thefurnace boat is placed in the tube 10, is purged, and is heated to 500°C. A source of H₂ gas is caused to flow in the quartz tube. When theequilibration is reached, the slider is moved with respect to the baseto position the well 15 with (Hg_(1-x) Cd_(x))_(1-y) Te_(y) charge 20over the substrate in recess 22 so that the epitaxial growth cancommence. The purpose of the wafer 26 is to give up Hg therebypressurizing the growth area, i.e., the region 27 under the cover 14, inthe wells 15 of the slider and in the groove or moat 17 around thebottom of the slider. Since this is a slider assembly, some Hg vaporleaks and diffuses out of the boat through the cover fit or through thesliding fit between slider section 12 and base section 13 and into theH₂ gas stream contained in the quartz tube 10. The Hg which escapes isreplaced by dissociation of the source wafer 26 to thereby maintain aconstant Hg pressure over the plug 21, and around the growth solution 20and growth substrate. Since the partial pressure of Hg from the sourcewafer 26 of HgTe is the same as the partial pressure of Hg from thecharge 20, i.e. about 0.1 atm, there is prevented any loss of Hg fromthe charge 20 and the LPE grown layer of Hg_(1-x) Cd_(x) Te on thesubstrate. The apparatus of FIG. 1 establishes the environment in whichthe improvement of the present invention is useful.

One limitation of the prior art is that some of the growth solutiontends to adhere to the surface of the HgCdTe epilayer after growth inthe slider apparatus. The present invention is an improvement over theprior art in the use of CdTe pieces to assist wipe-off of HgCdTefollowing LPE growth.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of the prior art.

FIGS. 2 and 3 show the invention.

DESCRIPTION

Turning now to FIG. 2, a "wipe-off" well 40 is incorporated into themoving part 12 of the slider apparatus. This well 40 holds severalpieces of CdTe 41 and 42 in vertical slots about 1 mm apart. The slotsare cut into the well as guides for the CdTe pieces. The CdTe pieces arepolycrystalline unpolished pieces which are a sliding fit in theirslots, so that they naturally rest against the upper surface of the basesection 13. As the CdTe pieces pass across the surface of the grownepilayer after growth is completed, they remove any residual growthsolution 43, FIG. 3, by mechanical wiping, by surface tension adhesionbetween the CdTe and the growth solution, and by capillary wicking ofthe growth solution into the space between the two CdTe pieces. Use ofthis wipe-off well 40 eliminates residual growth solution otherwise lefton the epilayer. The wipe-off well is pressured with Hg from the HgTereservoir 26 as was described for the other areas in discussion above.After cooldown, the two CdTe pieces are discarded as waste.

The embodiments of the invention in which an exclusive property or rightis claimed are defined as follows:
 1. In a covered graphite sliderapparatus for liquid epitaxy growth of mercury cadmium telluridecomprising:a furnace enclosure having therein a graphite furnace boat,the boat further comprising:a graphite base member having a generallyplanar top face and having a recess portion in the top face, the recessportion being adapted to receive a semiconductor substrate for epitaxialgrowth thereon; an improved graphite slider member having a planar lowerface adapted to mate with and to be slideable along said base member topface, said slider member having a first well extending therethrough froma top surface to the lower face for receiving and holding a charge of(Hg_(1-x) Cd_(x))_(1-y) Te_(y), said first well being slideable oversaid substrate for epitaxial growth of HgCdTe thereon, a second "wipeoff" well adjacent to said first well in said slider member andextending through said member to the lower face, which second well hasCdTe material loosely placed therein, said CdTe being slideable over theepilayer surface by movement of said slider member following LPE growthto pick up residual growth solution left on the epilayer surfaces, thesecond "wipe off" well further comprising vertical slots formed in thewell, said vertical slots acting as guides for CdTe pieces, said topsurface having a depression therein adapted to receive a mercurysupplying charge to said wells; and, a graphite cover positioned forcovering said slider member top surface.